Since the 1960s, the public switched telephone networks have utilized time-division multiplexed systems to carry both voice and data signals over a digital communications link. One of the earliest and most commonly used carrier systems for such signals is the T-carrier system. The T1 line is probably the most common type of T-carrier in use today. In North America, it typically provides a 4-wire transmission path that transmits bipolar pulse streams at bit rates of 1.544 Mbps. The four wires provide a full-duplex capability with one pair of wires for receiving and another pair for sending at the same time. Originally, the four wires were formed by two pairs of twisted-pair copper wires, but T1 wiring may now also include coaxial cable, optical fiber and other media. Standard digital signals carried on a T1 line are designated as DS-1 signals and must comply with certain timing, voltage level, and pulse shape characteristics.
Within the T-carrier system, higher data transmission rates are accomplished by grouping, or multiplexing, together lower-rate signals. For example, DS-3, the signal on a T-3 carrier, carries 28 DS-1 signals for a transmission rate of 44.736 Mbps.
Initially envisioned as primarily a link for voice and data signals between central offices on the telephone network, T-carrier facilities are now commonly being used to provide high-speed data services to customer premises. In this latter role, T-carrier signals are provided between a central office and a multiplexer/demultiplexer at a customer's building. At this location, a digital signal cross-connect (DSX) is typically present to pass signals between the multiplexer and one or more local transmission facilities. The DSX specifies the electrical interface required for connecting equipment which will communicate using a particular signal type. For example, DSX-1 defines the set of parameters needed for connecting lines carrying DS-1 signals. This relatively new use of transmission facilities has created the need for additional outside plant equipment as well as additional functionality in the equipment that is currently in place. The need for repeaters with expanded capabilities to interface between a DSX-1 network and a local span network at a customer's building is one example of such equipment.
However, T1 repeaters, as are known in the art, include many shortcomings which make them impractical and insufficient to operate within this new environment at or near a customer's building.
Repeaters utilize signal regeneration to accomplish their function of shaping signals. Regeneration involves receiving a distorted, weakened, or attenuated digital signal and reconstructing it so that its amplitudes, waveforms and the timings of its elements are constrained within specified limits. One family of repeaters, known as extension repeaters, regenerate signals in only one direction; the particular direction depending on the orientation of the repeater. Even though these repeaters are connected to each direction of the bi-directional facilities, they are designed with regeneration circuitry for only one direction. They are typically used in locations where certain assumptions about the adjacent equipment down the line are known to be true; with one of the assumptions being that the adjacent equipment in one direction can tolerate attenuated signals and has its own regeneration capability. Therefore, in such a case, the extension repeater does not need to provide regeneration in that direction. The most common use of these repeaters are within and between central offices where the connected equipment at each link is known and is controlled by the same parties who provision the repeaters. An installer of an extension repeater simply orients the repeater so that the regenerated output signal is provided in the correct direction. The prior art extension repeaters fail, however, to provide simultaneous signal regeneration in both directions.
Within the inter and intra office environments using T-carrier facilities, “line” repeaters are known which do have bi-directional signal regeneration capability. Because of the limited use intended for these repeaters, however, they include only very basic loopback and diagnostic capabilities. A loopback refers to a point in a communications network at which an incoming signal is “looped” and returned along its transmit path. Loopbacks are typically used to troubleshoot a network problem by verifying that the connection path between two devices in a network is functioning correctly. Previous line and extension repeaters lacked extensive loopback capability. In particular, they failed to provide selectable loopbacks toward the DSX-1 network, toward the local network span, and both sides simultaneously
Another practical shortcoming of current extension repeaters is their physical size. Because of the central office environment in which they were historically utilized, extension repeaters are relatively large and require rack mounting shelves to accommodate them. To compound the problem, such repeaters were, and are, rarely utilized individually but are rather routinely grouped together in large multiples such as 28 for handling DS-3 signals. While this size and aggregation of repeaters may be acceptable in the central office environment, the existing equipment racks and shelves for these repeaters are far too bulky and impractical for the typical telecommunication equipment room present in most customers' buildings.